1. Field of the Invention
The present invention concerns a detuning circuit unit for antennas in a magnetic resonance apparatus.
2. Description of the Prior Art
Magnetic resonance technology is a known modality to acquire, among other things, images of the inside of the body of an examination subject. In a magnetic resonance (MR) apparatus, rapidly switched gradient fields are superimposed on a static basic magnetic field B0 that is generated by a basic field magnet. To excite MR signals, the MR apparatus also has a whole-body radio frequency antenna (RF antenna) that radiates RF signals (known as the B1 field) into the examination subject. The excited magnetic resonance signals can be acquired by the antennas that emit the aforementioned signals, or by other antennas such as local antennas. The received signals are processed by a computer to reconstruct an image.
Antenna arrays are typically used in MR tomography. An antenna array has a number of individual coil elements. An example of an antenna array is a spinal column surface unit, also called a spine array. In such an antenna array with, for example, eight rows of three coil elements, four rows (twelve coil elements), for example, cover the maximum acquisition volume (field of view, FOV).
If coil elements are not necessary for particular examination, they are detuned with regard to their resonance frequency with the use of detuning circuits. For example, receiving antennas are switched to be inactive during the transmission mode and transmitting antennas are switched to be inactive during the reception mode. The activation of such a detuning circuit typically occupies a control signal line in the plug of the antenna unit. In conventional detuning circuits, a certain forward (on-state) current typically is provided by the MR apparatus in the detuned state (inactive state) via a PIN diode. In the active case, i.e. without detuning, a blocking voltage is applied to the PIN diode.
In addition to the transmission by means of a whole-body antenna mentioned above, a B1 magnetic field an also be radiated with good spatial localization with a local antenna array.
A problem with the use of a number of coil elements in an antenna array is that the number of control signal lines for detuning is limited by the physical size of the plug.